feat(gitsync): auto-bootstrap tom wiki från remote live-gren

Ett repo utan commits adopterar origin/<live_branch> automatiskt vid
första bakgrundssynken — färsk instans behöver ingen manuell reset.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/internal/git/sync.go

@@ -170,6 +170,24 @@ func (r *Repo) Push(branch string) error {
 	return err
 }
 
+// HasCommits reports whether the repository has any commit (false on
+// a freshly initialised, unborn branch).
+func (r *Repo) HasCommits() bool {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	_, err := r.run("rev-parse", "--verify", "HEAD")
+	return err == nil
+}
+
+// RemoteBranchExists reports whether origin/branch is known locally
+// (run Fetch first for a fresh answer).
+func (r *Repo) RemoteBranchExists(branch string) bool {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	_, err := r.run("rev-parse", "--verify", "refs/remotes/origin/"+branch)
+	return err == nil
+}
+
 // MergeInProgress reports whether a merge has been started but not
 // yet committed or aborted.
 func (r *Repo) MergeInProgress() bool {